Accurate measurement of cement slurry density has long been a problem where the slurry is being pumped over some distance. At source, the density can be calculated by weighing a known volume, but where the slurry is then subjected to compression and acceleration by pumps, and/or supplied to an area of different ambient pressure, the density at destination is often only approximated relative to density at the source.
This is particularly problematic in undersea operations, such as in the grouting of oil/gas platforms to the seabed. Such operations often require the pumping of cement slurry to the sea bed, typically in depths of hundreds of meters, and indeed to locations at least tens of meters below the seabed.
Knowledge of the slurry density at these depths allows calculation of the required slurry volume. This, in turn, can provide an indication of the integrity of the grout around a platform leg. Approximations of density calculated from the surface are simply not sufficiently accurate to provide this indication.
In order to provide a more accurate measurement of slurry density at the destination, it is known to use a remote nuclear-source densimeter. A nuclear-source densimeter uses a radioactive source of gamma rays such as caesium or americium. These are directed through the slurry to a detector, which calculates density based on the attenuation of the gamma rays.
The use of nuclear-source densimeters can be problematic. In particular, they can expose workers to gamma rays, presenting safety risks. Additionally, they can be expensive, and difficult to recover from an undersea cement pouring or grouting operation.
Other techniques for determining cement density have been used. In particular, coriolis flow meters are often used. These too are problematic as they require cement to be forced through narrow tubes in order for its density to be calculated.
Other techniques attempted include the use of strain gauges attached to tubes in an attempt to determine the weight of the tube. As with coriolis flow meters, this requires the forcing of cement through narrow tubes. The measuring of differential pressures has also been attempted, however this has proved impractical.
All of the techniques discussed share a further limitation, in that they can only measure density of a cement slurry within a pipe. Although knowledge of the slurry density as it exits a pipe below the sea bed is useful, it leaves open the possibility that due to cavities (caves) beneath the sea bed, and mixing of the slurry with mud, the actual density of the slurry in situ is significantly different to that of the slurry exiting the pipe.
The present invention arose from the realisation that density could be measured by the acoustic impedance of a slurry to ultrasonic waves. Attempts were made to measure this impedance by passing a wave through the slurry to a receiver. These attempts proved fruitless, as the heterogeneous nature of a cement slurry and the presence of so many suspended particles resulted in scattering of the waves and quick attenuation. Further research was conducted in order to arrive at the present invention.